This invention relates generally to cellular telephone communications, and more particularly to a system and method for mobile stations to use the pilot channel timing information to combine the various multipath signals from a base station.
Spread spectrum communication techniques allow communicating users to operate in noisy radio frequency (RF) spectrums, and are especially effective against narrow-band interferers. Spread spectrum communications can be effected at relatively low power spectral densities, and multiple users can share the same frequency spectrum. Further, receivers can be designed to protect against multipath. These system characteristics encouraged early development of the technology by the military.
Common forms of spread spectrum systems include chirp, frequency hopping, and direct sequence or pseudonoise (PN). The chirp system transmits an impulse signal in the time domain that is spread monotonically in the frequency domain. A receiver converts the spread frequency signal back into an impulse signal. These frequency-spread impulse signals have applications in radar, for the pulse position modulation of information, or both, such as the R.sup.3 transponder developed by General Dynamics, Electronics Division in the 1970s. Frequency hopping systems communicate by synchronizing users to simultaneously change the communication frequency.
Direct Sequence systems spread a digital stream of information, typically in a quadriphase modulation format, with a PN code generator, to phase shift key modulate a carrier signal. The pseudonoise sequence of the PN code generator is periodic, and the spread signal can be despread in a receiver with a matching PN code. Direct Sequence systems have excellent immunity to noise. The PN codes used typically permit a large number of users to share the spectrum, with a minimum of correlation between the user's PN codes. However, Direct Sequence system require large RF bandwidths and long acquisition times.
The IS-95 standard defines key features of the so-called second generation code division multiple access (CDMA) communication system, a type of Direct Sequence spread spectrum modulation. To help solve the problem of long acquisition time, the IS-95 signal uses a pilot channel. Each base station transmits a pilot channel message spread with PN codes known to all the mobile stations. The PN code is made up a series of phase shifted binary symbols called chips. The PN period is 32,768 chips and the PN chip rate is 1.2288 Megahertz (Mhz). The digital stream of information that is spread by the PN code is also known to the mobile stations. Because there is no ambiguity in the demodulated message, the timing characteristics of the PN code, down to the chip phase, as well as the QPSK modulation phase are known to the mobile station receiver.
The IS-95 system communicates information from the base station to the mobile stations through a series of traffic channels. These traffic channels are transmit and receive information, i.e. digitized audio signals, spread with a traffic channel PN code, unique to each mobile station. Using this precise timing and phase information derived from the pilot channel, the mobile station is able to acquire a setup channel, and eventually, the overall System Time. With this System Time, the mobile station is able to differentiate between base stations and synchronize the demodulation circuitry with sufficient accuracy to recover the received traffic channel message.
A third generation, wideband CDMA (W-CDMA) system is in development as described in "Wideband-CDMA Radio Control Techniques for Third Generation Mobile Communication Systems", written by Onoe et al., IEEE 47.sup.th Vehicular Technology Conference Proceedings, May 1997, that may have global applications. Instead of a pilot channel, the W-CDMA system has a broadcast, or perch channel. Each timeslot, or slot of the broadcast channel consists of a series of time multiplexed symbols. A long code masked, or special timing symbol segment uses just a short code to spread one symbol of known information. This segment allows a mobile station to acquire system timing information immediately after turn-on. The pilot, or reference symbols are similar to the IS-95 pilot channel. In one proposal, 4 reference symbols, with each symbol being 2 bits, are spread with a long code and a short code. The reference symbol information and the short code are known by the mobile stations. The long code is unique to each base station, so that timing information is refined, once the long code is known (the base station is identified). Therefore, according to some proposals, 5 symbols in the slot would be dedicated to the mobile station acquiring timing information. Further, both the long and short codes spread 5 symbols of data during each slot. Since information is not predetermined for the data symbols, precise timing information cannot be accurately recovered, as with the other two kinds of (timing) symbols. Other combinations of reference, special timing, and data symbols are also possible.
The W-CDMA system also includes several traffic channels to communicate information, such as a digitized voice or data. The traffic channel predominately includes information, but may also include a reference symbol segment. For example, at a data rate of 32 kilosymbols per second (ksps), a slot could include 4 pilot symbols and 16 information symbols. Precise timing information can be derived during the reference symbols segment of the traffic channel message, but not during the information segments.
Implementations of a RAKE receiver employing separate despreading circuitry for each detected path can be highly complex for multiple coded CDMA signals. Furthermore, implementing a RAKE receiver for such signals requires either separate sets of weights for each despread CDMA signal, or for a single set of weights to be multiplexed and subsequently weighted with the various despreader outputs.
The task of decoding CDMA signals is simplified with the use of matched filters, such as the matched filter to used to decode pilot symbols in the W-CDMA system. Analog discrete time signal processing does not consume a large amount of power, relative to digital signal processing (DSP) implementations. Such analog discrete time signal processing elements include delay elements, multipliers, summers.
Co-pending patent application, Ser. No. 09/015,424, invented by Kowalski, Kandala, and Somayazulu, entitled SYSTEM AND METHOD FOR CDMA CHANNEL ESTIMATION, filed on Jan. 29, 1998, and assigned to the same assignees as the instant application, discloses a procedure for using timing derived from the perch channel in a wideband CDMA system to despread and demodulate the traffic channels. Although the system simplifies the operation of the traffic channel, a traffic channel RAKE section is still required for every multipath signal.
Gilhousen, et al., U.S. Pat. No. 5,109,390, disclose a spread spectrum receiver capable of differentiating multiple pilot signals and selecting the signal of greatest strength. The transmitting base stations are synchronized to operate from a master clock. Receiving mobile stations can maintain timing accuracy sufficient to demodulate received messages from all base stations by monitoring the pilot channel of any single base station. However, Gilhousen et al. do not disclose a method of reducing the number of traffic channel RAKE sections, or an efficient manner of summing the traffic channel data.
Lomp, U.S. Pat. No. 5,673,286, discloses a system of fixed delay sections used to combine despread traffic channel messages to increase the signal to noise ratio of the received signal, while simplifying the receiver section. However, the system sill requires a significant number of traffic channel RAKE sections. The use of fixed delays does not permit the capture all possible multipath variations of a transmitted signal. Further, the use of fixed delay sections introduces noise into the recovered signal when a multipath variation of a signal is not acquired by the system.
Prior art descriptions of the RAKE matched filter, employing a finite impulse response filter prior to despreading, do not detect more than a single distinct signal path. Moreover, the prior art does not describe a means for controlling the use of tap weights (delay) in the signal output, resulting in an undesirable increase in noise at the combiner output. In addition, these RAKE weights consume chip real estate and power. It would be desirable to utilize a RAKE receiver in which energy was combined only from those delays in which a signal was actually detected, consistent with a minimal use of hardware resources.
It would be advantageous if a CDMA receiver design could be simplified to reduce the number of parts and decrease the receiver's power consumption.
It would be advantageous if the number of traffic channel RAKE sections could be reduced without degrading the signal to noise ratio of the receives signals.
It would be desirable for a CDMA receiver architecture to be able to utilize either discrete time analog signal processing, or to use digital signal processing and discrete time analog signal processing together.
It would be advantageous if the channel estimates derived from the pilot channel could be applied to all the received channels of the same transmission path. In this manner, channel estimation need only be performed once.
It would be advantageous if the traffic channel multipath signals could be combined using the timing information derived from their corresponding pilot channel signals. It would be desirable if CDMA signals, other than the pilot signals, could be despread with a single despreader circuit. That is, it would be advantageous if the traffic channels could be delayed in response to the pilot channel timing information, and then combined in a single traffic channel signal.
Accordingly, in a wideband wireless communication system where base stations transmit information to mobile stations, and the communications from a base station to a mobile station are propagated along multiple transmission paths, with each transmission path having a corresponding path delay, a method for combining received communications from the multiple transmission paths is provided. The method comprising the steps of:
a) routing the received communications of each transmission path in parallel; PA1 b) variably adjusting the delay of each parallel routed communication, compensating for the timing differences between transmission paths, whereby all the path delays are made equal; and PA1 c) summing the communications with compensated path delays, whereby the communications from the multiple transmission paths are combined to improve the signal to noise ratio of the communications. PA1 b.sub.1) variably adjusting the weight of each parallel routed communication in response to the step of calculating the weights, compensating for the signal strength differences between identified transmission paths. In this manner, the signal to noise ratio of the combined communications is maximized.
Prior to Step a), each transmission path of the received communications is identified and a time delay associated with each identified transmission path is calculated. Then, Step b) includes adjusting the delay of each transmission path in response to the delay calculated for each identified transmission path.
Each transmission path has a corresponding received signal strength, therefore, the step of calculating the time delay associated with each transmission path also includes calculating a weight associated with the received signal strength of each transmission path. A further step follows Step b), and preceding Step c), of:
In one aspect of the invention, the communications system is a code division multiple access (CDMA) system and the communications from a base station to a mobile station are formatted in a plurality of channels, including a pilot channel spread with a code, and a traffic channel with time multiplexed data symbols spread with code, and in which the step of identifying the transmission paths includes despreading the coded pilot channel.
The step of calculating a time delay for each transmission path includes generating a despreading code which is synchronized with the received communications to despread the pilot channel. A transmission path delay is generated in response to synchronizing the despreading code. Then, Step c) includes combining the traffic channel data symbols before they are despread. A following step despreads the traffic channel data symbols summed in Step c).
A time-varying finite impulse response (FIR) filter for combining received communications from a plurality of transmission paths is also provided comprising a plurality of variable delay circuits, a plurality of variable weighting circuits, and a combiner. Each delay circuit corresponds to a transmission path and has a signal input to accept received communications, a control input to set the delay through the delay circuit, and a signal output to provide the delayed communications. In this manner, compensation is made for the timing differences between identified transmission paths, and all the path delays are made equal.
Each weighting circuit corresponds to a transmission path and has a signal input operatively connected to a corresponding delay circuit signal output. A control input sets the weight, and a signal output operatively connected to a corresponding combiner input provides the weighted communications. In this manner, compensation is made for the signal strength differences between identified transmission paths, and all the signal to noise ratio of the combined communications is maximized.
The combiner has a plurality of inputs, with each input operatively connected to a corresponding weighting circuit signal output. The combiner has an output to provide the summation of the plurality of transmission path communications. The signal to noise ratio of the received communications is improved by combining all transmission paths.
Typically, time-varying FIR is part of a receiver for combining received communications from a plurality of transmission paths. When the communications system is a CDMA system, and the communications from a base station to a mobile station are formatted in a plurality of channels, including a pilot channel spread with a code, the receiver further comprises a plurality of pilot channel RAKE receiver sections with each pilot channel RAKE receiver section corresponding to a transmission path. Each pilot channel RAKE receiver section accepts CDMA communications, identifies a transmission path, and generates a despreading code to despread the pilot channel of the identified transmission path. The despreading code is used to derive path delay and to provide the path delay on an output operatively connected to the corresponding delay circuit input control. In this manner, the path delays for each transmission path are calculated in response to despreading the pilot channel, and used to control the delay circuits.
Likewise, each pilot channel RAKE receiver section measures the signal strength of its identified transmission path to determine a weight. The pilot channel RAKE receiver section has an output operatively connected to the corresponding weighting circuit input control to provide weighting in response to the transmission path signal strength. The signal strengths for each transmission path are calculated in response to despreading the pilot channels, and used to control the weighting circuits.
The receiver includes a traffic channel despreading circuit having an input operatively connected to the combiner output. The traffic channel despreading circuit has an output to provide the despread data symbols combined from a plurality of transmission paths.
Each delay circuit of the time-varying FIR further comprises a plurality of delay subcircuits each having a predetermined fixed delay. Each delay subcircuits has an input to accept communications, and an output to provide delayed communications. Each subcircuit is serially connected, with a preceding subcircuit output being operatively connected to a following subcircuit input. The delay circuit also comprises a switch having a signal input selectively connectable to one of the delay subcircuits outputs. The switch has an input control to select the operative connection between the switch signal input and one of the subcircuit outputs. The switch has a signal output to provide the delayed communication. The communications through the variable delay circuit are delayed in finite steps with the use of the delay subcircuits and switch.